1. Field of the Invention
The present invention relates to a braking force distribution control system for controlling a braking force applied to a rear wheel in a braking operation of a vehicle, and more particularly to the control system for controlling the braking force applied to the rear wheel in a certain relationship with the braking force applied to a front wheel.
2. Description of the Prior Art
In general, when a running vehicle is braked, axle loads applied to the front and rear portions of the vehicle respectively will be different from each other due to the moving load caused by the braking operation. Therefore, the braking force applied to a front wheel and the braking force applied to a rear wheel for locking all the wheels simultaneously are not in direct proportion to each other, but in such a relationship as indicated by a one-dotted chain line in FIG. 13. This relationship is known as an ideal braking force distribution which varies depending upon the condition with or without load. The distribution under the condition with load will be the one as indicated by a two-dotted chain line in FIG. 13.
If the braking force applied to the rear wheel exceeds the braking force applied to the front wheel, the directional stability of the vehicle will be deteriorated. In order to keep the braking force applied to the rear wheel lower than that applied to the front wheel and provide a distribution in close proximity to the ideal braking force distribution, a proportioning valve is provided between the rear wheel brake cylinder and the master cylinder. With this arrangement, a distribution characteristic has a break point as indicated by a phantom line in FIG. 13. When the difference of the loads applied to the inner and outer wheels of a turning vehicle is taken into consideration for example, it is necessary to reduce the braking force applied to the rear wheel much lower than the braking force applied to the front wheel. In addition, when the loadage is large, the distribution will be far remote from the ideal braking force distribution. Therefore, a load sensing proportioning valve is installed in a truck or the like to provide a distribution characteristic with a break point which varies in response to the loadage.
In Japanese Publication for Opposition No. 51-40816, it is proposed to vary the break point of the distribution characteristic for the proportioning valve by a pneumatic actuator, in accordance with a result of a comparison between the rotational speeds of the front and rear wheels. In that publication, is disclosed a structure which provides a high break point when the rotational speed of the rear wheel is higher than that of the front wheel, and provides a low break point when the rotational speed of the rear wheel is lower than that of the front wheel.
According to the above-described arrangement in the prior art, however, it is insufficient to provide a proximate ideal braking force distribution with respect to the front and rear wheels of the vehicle. Since the braking force distributed to the rear wheel is reduced in the prior art, a large force will have to be applied to the brake pedal for producing a desired deceleration of the vehicle. Otherwise, a large load will be applied to the front wheel braking system. Or, a large braking force will be distributed to the rear wheel, so that the rear wheel will be likely to be locked.